I have a manual gearbox I use for vintage circuit racing that calls for a GL-1 85w90 weight oil. It uses bronze synchro rings so I've always run such an oil so as to protect the rings.
My question is, would running a GL-4 spec synthetic 75w-80 with EP additives really damage the rings? Is there anything to be gained?
What are people's general thoughts on the matter?

Yes I read that paper a while back but it doesn't address the GL-1 spec at all.
It seems I can run GL4 according to the article and synthetic may even prove better, but still interested in people's experiences.

About 12 years ago I decided it was a good idea to adapt a Toyota R154 5-speed to a Ford 2.3 Turbo (long story). The R154's are known to be a good strong transmission, but they have simple brass syncro's unlike more modern transmissions. I made a custom bellhousing by cutting the back of a 2.3 bellhousing off and welding on a flange from a R154 bellhousing. Had it all lined up in a mill and machined straight, etc. Custom clutch, etc. I had all of the details worked out and put into the car. I had a Haynes manual for a Toyota Supra and it said use GL-3 or 4 in the transmission, so I did. The car was almost undrivable. You could not shift it at all. I thought I had screwed something up. Took the transmission out several times, put a dial indicator on the bellhousing, swapped clutches, made sure my custom shift linkage was working, etc. Nothing fixed it. Thought I had just wasted a lot of time.

Finally, out of desperation I put in some GM Syncromesh manual transmission fluid. Totally transformed the transmission, now shifted like butter. I couldn't believe it I then read up on how all of the newer gear oils have a much better high pressure additives that off course is counter productive to how syncro's work.

I've had mixed results with synthetic manual transmission lube and older transmissions. Some, a '65 Ford toploader, were greatly improved. Others, it was maybe too slippery and synchros couldn't do their job, resulting in some gear clash were there had been none with warm conventional lube.

But your bottom line, I've not seen any deterioration of the synchros themselves with the synthetic lube.

There are certainly some interesting stories out there....i guess i need to try it myself and see how it performs in this particular application. I also forgot to mention I've doubled the engines original power. So far its held together, i guess im looking for continued reliability and figured on the synthetic stuff.

Synchronizer rings should be BRONZE ('bohnalloy' was what GM specified). This is part of the problem for us folks that still use bronze ring transmissions. "Reproduction" parts, that as long as it kinda looks the part, are expected to function as the OEM required.

In addition, there are less and less oil manufacturers/distributors concerned with a PERFORMANCE bronze-ring transmission.

Technically:
All GL-4 non synthetics should be SAFE
All GL-5/MT1 oil should be SAFE

Neither of these guarantees that the gearbox will be able to shift well at all.

We recommend you use one of the following 3 oils, or at least pick an oil that specifically states BY name, the transmission you'll be using it in.
Brad Penn 80w90 GL4 "Classic Gear Oil"
Royal Purple 75w90 "Max Gear"
Over the past year, we've worked with Driven to provide a Gear Oil SPECIFICALLY for bronze ring transmissions. After extensive testing, their 80w90 GL4 has just been released "Driven GO conventional 80w90 GL-4"

If you don't know the quality of the synchro parts in the gearbox you're working with, I'd recommend either of the 80w90 products listed above.

autogear wrote:Synchronizer rings should be BRONZE ('bohnalloy' was what GM specified). This is part of the problem for us folks that still use bronze ring transmissions. "Reproduction" parts, that as long as it kinda looks the part, are expected to function as the OEM required.

In addition, there are less and less oil manufacturers/distributors concerned with a PERFORMANCE bronze-ring transmission.

Technically:
All GL-4 non synthetics should be SAFE
All GL-5/MT1 oil should be SAFE

Neither of these guarantees that the gearbox will be able to shift well at all.

We recommend you use one of the following 3 oils, or at least pick an oil that specifically states BY name, the transmission you'll be using it in.
Brad Penn 80w90 GL4 "Classic Gear Oil"
Royal Purple 75w90 "Max Gear"
Over the past year, we've worked with Driven to provide a Gear Oil SPECIFICALLY for bronze ring transmissions. After extensive testing, their 80w90 GL4 has just been released "Driven GO conventional 80w90 GL-4"

If you don't know the quality of the synchro parts in the gearbox you're working with, I'd recommend either of the 80w90 products listed above.

How about a synthetic GL-4? 75w 80 to be specific. Any thoughts? I hear synthetics may be even safer on yellow metal. To be honest I've never really seen yellow metal damage other than discolouration that I'd put down to the wrong oil.

autogear wrote:Synchronizer rings should be BRONZE ('bohnalloy' was what GM specified). This is part of the problem for us folks that still use bronze ring transmissions. "Reproduction" parts, that as long as it kinda looks the part, are expected to function as the OEM required.

In addition, there are less and less oil manufacturers/distributors concerned with a PERFORMANCE bronze-ring transmission.

Technically:
All GL-4 non synthetics should be SAFE
All GL-5/MT1 oil should be SAFE

Neither of these guarantees that the gearbox will be able to shift well at all.

We recommend you use one of the following 3 oils, or at least pick an oil that specifically states BY name, the transmission you'll be using it in.
Brad Penn 80w90 GL4 "Classic Gear Oil"
Royal Purple 75w90 "Max Gear"
Over the past year, we've worked with Driven to provide a Gear Oil SPECIFICALLY for bronze ring transmissions. After extensive testing, their 80w90 GL4 has just been released "Driven GO conventional 80w90 GL-4"

If you don't know the quality of the synchro parts in the gearbox you're working with, I'd recommend either of the 80w90 products listed above.

Autogear, I understand that lacking a front seal, stock Muncies (M21 in my case) tend to leak with synthetic oils. Would the Driven formulation be suitable for them?

Also, while we're talking Muncies, do newer oils like the above provide any better odds re seizing first gear in road racing apps? (I hope to run a couple of track days with my BB '70 1/2 Camaro when I get it back on the road.)